F-J's Physics - Barton's Pendulums and Resonance - Video 35 

Pleasure! So pleased it helped. Good luck with your studies and, as ever, enjoy your physics!

Mohsen - your English is excellent and way better than my Farsi! So pleased that you found it helpful and thank you for taking the time to comment. Enjoy your physics!

Oh am persian too

Many thanks and really pleased that you enjoyed it and found it useful. I had fun making it. It is an interesting experiment.

@@AnthonyFrancisJones you sir you genius Please don't stop making it.. Do with some qm

Thanks - I will try to keep them coming - do suggest topics and if it fits with the sort of thing I do I can give it a go.

@@AnthonyFrancisJones try some magnetism

Think I have done a few now!

Pleasure and thank you for taking the time to comment. It's a lovely bit of physics and has so many applications. I felt that it might have been a bit of a 'forgotten experiment'. As ever, enjoy your physics!

Yes, it is a great experiment. Why don't you try and build one - really simple. Thanks for your very kind comment and so pleased it helped you with your physics.

Thank you and great that you found it helpful. Thanks for taking the time to comment and wonderful to hear all the way from Sri Lanka!

Thanks for you kind comment - so pleased you found it useful.

Pleasure - Glad it helped.

Absolute pleasure - glad you found it useful!

Thank you - that's really kind. I never think of myself has having an accent but I know what you mean! Pleased that the video helped. It is a great experiment and good luck with all your physics studies and thanks for watching!

My pleasure and I am so pleased that it has helped with your studies. Keep up the great work! It will be worth it!

Thanks! Glad you found it useful and good luck with your studies.

Ember - so pleased it helped. Not an easy topic but a nice little experiment. Do continue to enjoy your physics.

@@AnthonyFrancisJones I sure will :)

Great - thanks for commenting and glad you found it useful.

Thanks, so pleased it was helpful - I think it is a bit of a forgotten experiment but I am glad that people still know about it and take an interest.

My pleasure. Glad you found it useful and it helped with your understanding of physics. Plenty more on my channel should you need more help!

That's very kind of you and great to hear that you are watching in Sri Lanka - As I always say, 'Enjoy your physics'!

@@AnthonyFrancisJones I've a doubt, Instead of small cones, why don't we consider heavier weights more appropriate ? Even when measuring gravity using a pendulum, we use bobs around 100g, why is that ?

@@onelaperera7936 Good question. So you will notice the weight that gets the cones oscillating is quite a large mass compared with the cones so it stores a lot of energy. The cones are light so the slightest amount of energy transferred to them makes them move - they only need a small amount of gravitational potential energy to swing high up - heavy ones would need much more to get to the same height. Now, measuring g you want to eliminate the effects of drag/air resistance so make the mass of high density and small size so a brass bob or similar is excellent for this. Hope that helps explain it!

@@AnthonyFrancisJones yes!understood, THANKYOU SIR

@@onelaperera7936 Pleasure!

Pleasure - hope you found it useful.

Thanks for taking the time to comment and glad you found it useful and interesting.

Min Linting - thanks for letting me know - it is a lovely experiment and every physics student should see it. Hope you enjoy some of my other physics videos too. Good luck with your studies.

Thanks, Glad you liked it. It is a great experiment!

Thanks Suzan - so pleased that it helped. Lovely experiment this one and I did not get to teach it this year due to schools being closed.

Adrian - glad you liked it - seems to be a popular topic this one.

Pleasure - glad you enjoyed it and found it useful.

Thank you. I am really pleased to hear this. Again, a simple experiment but tricky physics behind it! Good luck with your studies!

Great - so pleased it helped you understand what is not an easy practical demonstration to get the hang of. Hope your studies continue to go well!

Pleasure. Glad you enjoyed it and hope it was useful. It is a lovely demonstration.

Pleasure - glad you found it useful!

My pleasure Dulmi - so pleased you found it useful. It is a lovely experiment. As ever enjoy your physics!

Pleasure - glad you enjoyed it and it helped you - it's a nice demonstration that everyone should see.

@@AnthonyFrancisJones yes! also I wanted to say that today I had physics exam and there was a question exactly about this experiment, and I could easily answer it because of this video! I'm so thankful!

@@tinataheri4559 That's brilliant! So pleased! It is easy to build so all schools should have one to demonstrate. So much physics in it I thought I had overdone it in the video but it's too good not talk about! As ever, enjoy your physics and hope the videos help. I must publish all my ones on OCR topics such as Gravitational Fields and Waves as well as how to handle data.

Thanks for your kind comment. Careful with the phase bit - if they were 180° out of phase the displacements would add to zero! So they are 90° out of phase. Think of pushing a child on a swing. When they are at the top of the arc - stationary, you move most to push them. It is a similar situation here. I have not gone into the maths or the graphs of displacement, velocity and acceleration/time but it is worth looking at these too. Hope that helps and thanks so much for watching!

Pleasure - glad you liked it!

Thanks - so pleased you found it helpful. Thanks for taking the time to comment.

Thanks Reshma. This seems to be an experiment that is seen as very important to understand in India. I am not sure so many people in the UK see it but I am so pleased it helped and good to hear that physics remains strong in India! Good luck with your studies.

Thanks, hope you found it useful.

@@AnthonyFrancisJones my teacher suggested me your video...!!!

You must have a great teacher! Thank them on my behalf and hope your studies are going well!

Excellent - so glad it helped.

Thanks - glad you like it - another one that was not that easy to film.

You cover very insteresting contraptions, I first found you from the balancing bird, keep up the great videos!

Thanks - the balancing bird was my first and strangely one of the most popular. I plan to do one a week for a year and then see what happens after that - 52 is quite a lot but it is fun and I am so pleased people enjoy them. If there is one you would like me to do let me know and I will give it a go if I can.

Pleasure - glad you found it useful.

Thanks. Glad you liked it and found it helpful. Good luck with your studies.

Excellent Henry! So pleased it helped. I guess you are ending your A Level this year but if you need it there are full lessons on Gravitational Fields, Electric Fields, and Waves in my Corona Buster playlist. Thanks for taking the time to comment and good luck!

@@AnthonyFrancisJones Nope! I'm in year 12, it just our teachers are desperately speeding through everything so we can then go back onto the stuff that need to be re-learned or just properly understood. I may become a regular visitor here for the next year haha, thanks

@@henrygavin6034 Great - glad it is helpful. I do hope to do more A Level stuff - I have done a series on Waves for AS level in the Corona Buster playlist - just me teaching but my A Level pupils found it useful during lockdown and they also used all the lessons on Gravitational Fields too. Thanks for taking the time to comment and best of luck with your physics!

Thanks Ramya - glad you enjoyed it and found it useful. Do have a look at some of my other videos if you like and as ever enjoy your physics!

Good question. They vibrate at a range of frequencies (dependent on their lengths) all close to that of the periodic force. The one that vibrates the most (highest amplitude) vibrates at the frequency of the periodic force (or multiples of it). Have a look at 'resonance curves" that might help you understand better what is happening. Thanks for the question and I hope that helps.

@@AnthonyFrancisJones Thankyou very much Sir 🙏🏻.

Good question - yes it would cause the heavy mass to begin to oscillate as expected but it would not have as much initial energy as it is lighter so the movement of the heavy mass would not be as large. But, as you guessed, they will have similar natural frequencies so energy will move between them. Perhaps you can see now why we use a heavy mass to drive the system instead of having a second paper cone of the same length. Great question and thanks for asking!

Good question. On this scale the distance from the driver makes little difference. You need to have a look at 'resonance curves' which show that as the length gets further from the resonant length the amplitude (once 'settled down') is lower. The order of the strings here does not matter either, it is just they are typically put in reducing length order so you can almost see the shape of an amplitude vs length curve! Hope that helps and thanks for watching.

@@AnthonyFrancisJones Thank you for your reply! To clarify, by " as the length gets further from the resonant length," do you mean only a length greater than the resonant can have the lowest amplitude, or just the one with the biggest difference in length (compared to the resonant length) ?

@@randomhumand4757 As the length gets smaller or larger than the resonant length (length of the driver) the amplitude reduces. It is not a linear relationship but have a look at resonance curves and you will see. It applies in lots of situations from swings to tuning circuits in radios. So as you say the lowest amplitude will typically be the length with the biggest difference in length, but before I get corrected it is not quite symmetrical around the resonant length with amplitudes dropping off more rapidly with higher frequencies. Google resonance curve and it will help explain things even better!

Great - I think it is a bit of a forgotten experiment but of course the physics behind it is frequently asked about in exams usually in a 'real life' situation. I had a hunt for the question but I could not find it. I would be interested to see it. Those exam papers are excellent.

Ok, so I will try my best taking your questions in order. The non-resonant pendulums are not at the same frequency (they are non-resonant). They are almost there but not quite - too high or too low compared with the swinging mass and the single resonant one. Being in phase means, in simple terms, swinging together side by side without every changing that line up as it were. They all are not doing this as they are all swinging at different frequencies to each other and the swinging (driver) mass. So if the force acting on on of the pendulums is such that it 'pushes' against the direction of travel that pendulums amplitude (highest swing point) will get less and less. Finally you say, "only the driver and second to longest ones are resonating because they have the same natural frequency and so have the largest amplitudes" is correct. Do hope that helps and thanks for a great set of questions. Not an easy bit of physics!

​@@AnthonyFrancisJones Thanks for your quick response! I have learnt that when an external periodic force acts upon a vibrating body, the vibrating body acquires the frequency of the external periodic force gradually irrespective of whether its own natural frequency is equal to the frequency of the external force. (forced vibration) if its natural frequency is equal to the frequency of external force, the body resonates. if its natural frequency is unequal to the frequency of external force, the body oscillated at a small amplitude. This is what first led me to the assumption that the non-resonating pendulums are forced into vibration at the frequency of the driver but vibrate only with a small amplitude because their natural frequencies are not equal to the one of the driver but as you said, their frequencies are almost there but not quite...

@@thecomplexity Yes, that's right. All the pendulums feel a periodic FORCE at the frequency of the driving mass but do not move AT the same frequency unless they are resonant and even then the oscillation is 90 degrees out of phase. Now there is a situation where two 'distant' (very very lightly coupled) systems such as clocks on a wall or metronomes will at first be ticking out of phase but will very slowly move in phase but they of course have the SAME natural frequency. Perhaps this is what you were thinking of. Anyway hope some of this helps and good on you for asking!

Good question - yes it is oscillating at its natural frequency as it is a 'free' not 'driven' oscillation so by definition it is at its natural frequency. Yes, it is slightly damped but one can consider that that is only an energy (amplitude) loss. The pendulum provides the periodic force to the other masses - paper cones. They then demonstrate resonance where the periodic frequency they are driven at is similar to their own natural (or resonant) frequency. Not an easy one to grasp! Hope that helps!

Excellent question Andra. So if I understand it correctly in terms of the situation you are explaining: To drive a system and make it absorb energy best you want to be oscillating with a pi/2 phase shift - you are right about that. However, if you want to dampen (reduce the oscillation) then you have to have a mass move out of phase with the mass you want to damp in many cases - called Tuned Mass Dampers or TMDs. Taipei 101 is a great example of the use of a TMD. Not really a great explanation (and no maths) but I hope you get the idea that there are engineering solutions that use pi phase shifts. Hope that helps a bit and do enjoy your physics.

@@AnthonyFrancisJones Hi sir. Just a quick follow up. Doesn't damping imply the system dissipating energy? So if that's the case, why isn't the reduction of the oscillation at a maximum (critical damping I believe?) if the tuned mass damper is at a pi/2 phase shift, rather than a pi phase shift? If damping is due to the system dissipating energy, isn't it better for the tuned mass damper to absorb the most energy from the system? Thank you very much for your reply!

Sorry to ask so many questions. It's just that I am currently doing my physics project on (pendulum) tuned mass dampers, and would like to solidify my understanding. Thank you so much.

@@AndraHueiHsiaLye - pleasure - not sure I really helped but hope your project goes well! Let us know how you get on.

@@AndraHueiHsiaLye OK, I think the difference is between damping - which in many ways is what this video shows - the heavy mass has energy taken from it not by friction (only) but by the tuned cone wanting to swing at the same frequency but pi/2 out of phase. The pi phase damper moves (applies a force) in the opposite direction to an oscillating system to stop it moving rather than to reduce energy. So one system 'steals' energy, the other system is designed to stop it starting to oscillate in the first place. So two different situations. In the second system the mass that is stopping it oscillating is not just freely moving but is driven by hydraulic rams. In the first system there is no 'motor system' making it work. I have see these pi shift systems on the sides of large ships to reduce the oscillations in the waves. They do not take energy out of the system, they just stop the oscillation happening and building up in the first place. Hope that makes sense and thank you for your excellent questions. You learn and I have to think too - all good science!

Great! Hope the class found it useful. Your teacher should build one for you too!

Thanks for the question. Each length has a different natural frequency so swings at a different rate/frequency. Short is higher in frequency than longer - a bit like strings on a piano. If they swing at different frequencies there will be a phase difference between them. i.e they will not always be in line with each other. But the weight and one of the pendulums (of the correct length) will always be swinging together at the same frequency but with a difference in 'position' that remains the same between them all of the time. So they are not in phase either but have a constant phase relationship. Hope that helps.

Good question - all the heavy mass has to do is provide a vibration at one specific frequency. This will cause the others to vibrate and the one that is the same frequency will vibrate best (highest amplitude) as at resonance it absorbs energy from the heavy one best. Now the store of energy in the heavy one will depend on its mass and it is losing energy all of the time to keep the system going so the more mass (and therefore energy) it has the better. It would work with a lighter one but would not keep going for so long. Hope that explains it and thanks for your question.

@@AnthonyFrancisJones I learned this in university, I think heavy mass pendulum doesn't provide one specific frequency. It gives one specific Amplitude. And a nother question, according to forced frequency (heavy mass pendulum) , all the lighter pendulums oscillate same frequency (heavy mass pendulum frequency) ?

@@realphy6656 No, in both cases! The heavy mass is at one frequency but it can still cause others to oscillate just either side of resonance - yes, you could argue that it does provide amplitude as if it did not nothing else would move but not a specific amplitude - any reasonable amplitude will do as frequency is independent of amplitude! All the lighter pendulums do not oscillate at the same frequency - that is the whole point of this apparatus - they oscillate at frequencies near to the heavy mass but not at its frequency - that's why you see the phase shift between them all. Hope this helps and I have done a reasonable job of explaining it. I guess the real problem is I try to explain things without just using maths as a way of doing it!

@@AnthonyFrancisJones Thank you very much. I got this.

@@realphy6656 Great - it is not easy but once you get it it all makes sense!

Glad you liked the video and hope you found it really useful. Good luck with your studies!

Glad you enjoyed the film and I hope it helped you understand this interesting bit of physics!

Thanks - not quite sure of your drift. You cannot absorb vibrations - you can 'transfer the energy from the vibration's kinetic energy' if that is what you mean. In terms of opposition, there is a pi/2 phase shift so the oscillations are not really in opposition (I suppose that would be pi radians out of phase). It is an interesting demonstration and not that easy to explain fully. Coupling between oscillating systems is tricky at the best of times! Regardless, thanks for watching and taking the time to comment.

Thanks Yasir and good luck with encouraging others to learn more about the wonderful subject that is physics!

@@AnthonyFrancisJones Thank you dear Francis! :)